Composite laminates, also known as sandwich composites, are widely known for their low densities and high mechanical conformability. Generally, composite laminates feature two high tensile strength outer metal skins, and an intermediate core extending continuously and coextensively along the skins. In order to minimize weight, the core typically either is composed of foam or possesses a construction aimed at weight reduction, such as a honeycomb structure. Adhesives at each of the core-skin interfaces bond the skins and core together.
Processes of manufacturing sandwich composites generally involve the practice of a wet lay-up, lamination press, autoclave, or closed mold technique. An example of a lamination manufacturing process is depicted in FIG. 15. A movable top platen 202 and a bottom platen 204 of a press arranged in open, spaced apart relationship carry lay-up plates 206 and 208, respectively. To make laminate 210, outer skins 211, 212 are placed on the inner surfaces of lay-up plates 206, 208, respectively. Adhesives 214, 215 are either pre-coated on the inward facing surfaces of skins 211, 212 or applied to skins 211, 212 during lay-up in the press. Honeycomb or foam core 216 is situated between adhesive layers 214, 215. Then, platen 202 is moved downward to close the press, and platens 202, 204 are heated to a temperature sufficiently high to melt adhesive layers 214, 215. High pressure is applied by platens 202, 204 as adhesive layers 214, 215 are melted then subsequently allowed to cool. Once cooled, adhesive layers 214, 215 bond the opposite sides of core 216 to skins 211, 212, respectively.
While the above-described manufacturing process establishes bonding between skins 211, 212 and core 216, it is also responsible for introducing latent stresses into laminate 210. The latent stresses arise from the different coefficients of expansion possessed by the skin and core materials. Generally, metal skins 211, 212 contract less than core 216 during the cooling stage of manufacture. The disparity in contraction rates of skins 211, 212 and core 216 introduces a shear force at the core-skin interface, that is, along adhesive layers 214, 215, as pictorialized in FIG. 16. The setting of adhesive layers 214, 215 simultaneously with experienced shear forces at the core-skin interface builds a latent shear stress into laminate 210 along adhesion layers 214, 215, especially in continuous lamination processes where cooling is conducted at high rates. The incorporated stress may not be observable or revealed as a defect until long after manufacturing, such as when laminate is in use in the field.
Manifestation of defects caused by the latent stresses in laminate 210 may occur in the field as the result of vibratory energy or a traumatic force applied to laminate 210. In FIG. 17, for example, point A on skin 212 represents an impact region of a large vibratory force. Because core 216 is fashioned as a continuous film contiguous with skins 211, 212, vibratory energy imparted at point A passes directly from skin 212 to core 216, which transports the energy along the length and width of skins 211, 212. The propagation of the vibratory energy throughout laminate 210, combined with the intrinsic latent shear stress infused into adhesive layers 214, 215 during lamination, can overcome the bond strength of adhesive layers 214, 215 and cause one or both of skins 211, 212 to delaminate in part or entirely from core 216.
Similarly, FIG. 18 depicts the effect of a downward impact force on skin 211. The impact force shown in FIG. 18 is sufficient in magnitude to physically deform skins 211, 212 and core 216. While metal skins 211, 212 experience little or no change in thickness caused by the impact force, the continuous solid core 216 is compressed between skins 211, 212. Compressed core 216 displaces laterally outward at and beyond the impact zone. The lateral displacement of core 216, juxtaposed against the lesser effected skins 211, 212, generates a shear action at the skin-core interfaces, which already are infused with latent internal shear stresses described above. Consequently, the impact force may overcome the bond strength of adhesive layers 214, 215, causing one or both of skins 211, 212 to delaminate in part or completely from core 216. The extent of delamination can be compounded if laminate 210 experiences additional vibratory energy or physical impacts.
It is therefore an object of the present invention to provide a structural sandwich composite and a process of manufacturing the same that reduce or eliminate the above described drawbacks of known sandwich composites.